1. Field of the Invention
The present invention relates to a voice communication terminal which establishes a session and performs voice communication with a communication partner terminal through a packet switching network.
2. Description of the Related Art
Recently, various communication services have been provided by using IP (Internet Protocol) networks typified by the Internet. Such services include an IP telephone service using SIP (Session Initiation Protocol) as a call control protocol. The IP telephone service using SIP makes a source terminal transmit first an INVITE message as an outgoing call request message to a destination terminal. In response to this request message, the destination terminal (terminating terminal) returns an INVITE response message (status code 180 Ringing) if the destination terminal is in the standby state. Subsequently, this service executes a predetermined procedure to establish a session between the source terminal and the destination terminal, and allows the two terminals to make telephone speech between them upon establishment of a session. If the destination terminal is not in the standby state, the destination terminal transmits a response message containing the state to the source terminal. If, for example, the destination terminal is in the calling state, the terminal returns an INVITE response message (status code 486 Busy Here) indicating that the destination terminal is busy. For example, Jpn. Pat. Appln. KOKAI Publication No. 2004-304374 or Impress standard textbook series “Revised Edition SIP Textbook”, supervised by Yasubumi Chimura and Toshifumi Murata, Dec. 11, 2004 describes the details of this call control using SIP.
Assume that using SIP as a call control protocol, two terminals transmit outgoing call request messages (INVITE messages) at almost the same time. In this case, since the terminals are both in the calling state, the terminals return INVITE response messages (status code 486 Busy Here) indicating they are busy to each other. Consequently, the two terminals cannot establish a session with each other and cannot start any telephone speech.